Chromatograph mass spectrometers combining a chromatograph, such as a gas chromatograph (GC) or a liquid chromatograph (LC), and a mass spectrometer, such as a quadrupole mass spectrometer, are widely used to perform qualitative or quantitative analyses of various components contained in a sample. When performing a quantitative analysis of known compounds using a chromatograph mass spectrometer, an SIM measurement method is typically used which selectively and repeatedly detects only ions having one or more specific mass-to-charge ratios (m/z) that are designated in advance.
When a known compound is quantitatively analyzed using a chromatograph mass spectrometer including a chromatograph, such as a GC or an LC, and a triple quadrupole mass spectrometer, an MRM measurement method is used. According to this method, an ion (precursor ion) having a specific mass-to-charge ratio is selected by a first-stage quadrupole mass filter, the ion is then fragmented by collision-induced dissociation (CID) in a collision cell, and an ion having a specific mass-to-charge ratio among the product ions generated by the fragmentation is selected by a second-stage quadrupole mass filter, and the selected ion is detected. The MRM measurement method is advantageous in that the influence of irrelevant substances can be removed by the quadrupole mass filters at the two stages, so that the SN ratio of the detection signal is improved and a higher level of sensitivity is achieved in quantitative determinations.
When performing quantitative analysis through either SIM measurement or MRM measurement using a chromatograph mass spectrometer, the value of mass-to-charge ratios corresponding to target compounds is required to be set in conformity with the retention times of the target compounds as one measurement condition. For example, chromatograph mass spectrometers described in Patent Literatures 1 and 2 have a function of automatically creating a parameter table representing measurement conditions. After an analysis operator creates a compound table including information relating to measurement target compounds, the parameter table is automatically created based on the information described in the compound table. Such an automatic parameter table creation function according to a conventional chromatograph mass spectrometer will be described with reference to a specific example.
FIG. 8 illustrates an example of a settings input screen for allowing an analysis operator to set measurement conditions. As shown in the figure, a compound table display field 81 in which a compound table is displayed, and a measurement loop time input field 83 for allowing an analysis operator to input a setting value for a measurement point time interval, which is called a measurement loop time, are provided on the settings input screen 80.
The compound table displayed in the compound table display field 81 includes information of a compound name, a predicted retention time, a process time, a mass-to-charge ratio of a quantitative ion, and a mass-to-charge ratio of a confirmation ion for each target compound. The quantitative ion is an ion which best characterizes the compound. The confirmation ion is an ion which has a mass-to-charge ratio different from that of the quantitative ion and characterizes the compound. This confirmation ion is typically used to confirm that the chromatogram peak of the quantitative ion originates from the target compound by using the relative ratio between the signal intensity of the confirmation ion peak and the signal intensity of the quantitative ion peak on the mass spectrum. The retention time is a predicted value of the time of elution from a column in the liquid chromatograph. The process time is a parameter for designating a time range centering the predicted retention time during which the compound is to be measured, where an appropriate time margin is set so as to accommodate the variation in peak width and retention time. Accordingly, even if the retention time of a certain compound varies, the peak of the compound reliably appears within a range of the retention time of the compound±the process time. FIG. 9 shows the relationship between the peak of a compound on a chromatogram and the retention time and process time.
In the process of automatically creating the parameter table, a measurement time is appropriately divided into segments based on the compound table as described above. A segment is the smallest time unit for setting measurement conditions such as the conditions regarding the ions to be measured. The measurement conditions can be switched on a segment-by-segment basis.
In the process of automatically creating a parameter table, a boundary between segments is automatically set at a time point within an interval between retention times of compounds to be measured which is sufficiently large. More specifically, if a conditional expression,([the retention time of a compound X]+[A])<([the retention time of the compound X+1 eluted next after the compound X]−[A]) (where ±A is a process time)  (1)is satisfied, the segment boundary is set at a time point where the elution time range (retention time±A) of the compound X does not overlap the elution time range of the compound X+1, typically an intermediate time point between the retention time for the compound X and the retention time for the compound X+1, and thus the measurement time is divided into a plurality of segments by the segment boundary.
FIG. 10A and FIG. 10B show chromatograms for describing a segment dividing method. As shown in FIG. 10A, if the elution time ranges of the compound X and the compound X+1 overlap with each other, no segment boundary is set. That is to say, in this case, the compound X and the compound X+1 belong to the same segment. On the other hand, as shown in FIG. 10B, if the elution time ranges of the compound X and the compound X+1 do not overlap with each other, a segment boundary is determined at an intermediate point between the retention time of the compound X and the retention time of the compound X+1. Therefore, the compound X and the compound X+1 belong to different segments. According to such an algorithm, segments can be defined for all the compounds (or some compounds which are designated by an analysis operator) listed in the compound table.
FIG. 11 shows one example of a parameter table that is automatically created based on the compound table shown in FIG. 8. In the parameter table, the measurement conditions for one compound are listed together on one row, with each row including, besides the compound name, the number of a segment where the compound is measured (hereinafter, the segment number is indicated by “#”), the measurement start time, the measurement end time, the event time, the mass-to-charge ratios of the ions to be measured, and the dwell time. For the mass-to-charge ratios of the ion to be measured, the mass-to-charge ratio m/z−1 of the quantitative ion as well as the mass-to-charge ratios of the confirmation ions m/z−2 and m/z−3 of the compound to be measured are set. The measurement start time and the measurement end time are the start time and the end time of the segment in question. The event time is the time allotted to perform measurement once (this is called a “measurement event”) for one compound. The dwell time is the time during which the detector actually receives and accumulates ions, that is, the data collection time.
In the example in FIG. 8 and FIG. 11, a time interval sufficiently satisfying the conditional expression (1) exists between a compound C and a compound D subsequently eluting from the column. Accordingly, a segment boundary is set there. Segment #1 and segment #2 are created before and after the boundary, respectively. Further, since a time interval sufficiently satisfying the conditional expression (1) exists between the compound D and a subsequently eluting compound E, a segment boundary is also set there, and segment #3 is created after the boundary. Thus, compounds A to C are assigned to be measured in the time period of segment #1 whose measurement start time is 9.5 [min] and measurement end time is 10.8 [min], and the compound D is assigned to be measured in the time period of segment #2 whose measurement start time is 10.8 [min] and measurement end time is 12.0 [min]. FIG. 12 is a schematic diagram showing the correspondence between segments and compounds shown in FIG. 11, with time as the abscissa.
The event time is automatically calculated based on the setting value of the measurement loop time that the analysis operator input to the measurement loop time input field 83 of the settings input screen 80 and the number of compounds measured in one segment. FIG. 11 shows an example where the measurement loop time is set to 300 [msec]. In this case, the ions originating from each compound need to be measured at an interval of a measurement loop time of 300 [msec]. Since the compounds to be measured in segment #1 are the three kinds of compounds A to C, the event time allotted to each compound is 300 [msec]÷3=100 [msec]. In segment #2, the compound to be measured is the compound D alone, and hence 300 [msec] that is the same as the measurement loop time is allotted as the event time.
As described above, the dwell time is the time during which the detector actually captures ions. The event time includes, in addition to the dwell time, a wait time (hereinafter, called “voltage stabilization wait time”) which is required for the voltage to stabilize after the voltage applied to a quadrupole mass filter is changed. The dwell time also depends on the number of ions to be measured (number of measurement target ions) in one event time. Accordingly, the dwell time TD for each ion is calculated by the following equation (2).TD=([event time TI]−[voltage stabilization wait time])/[number of ions to be measured]  (2)
In the example illustrated in FIG. 11, the voltage stabilization wait time per ion to be measured is set to 1 [msec]. As a result, in segment #1, the dwell time TD for each ion is (100−1×3)÷3≈32 [msec]. Further, in segment #2, the dwell time TD for each ion is (300−1×3)÷3=99 [msec].